Answer:
They might have already been there after God made Adam and Eve.
Step-by-step explanation:
Some people say that God made the dinosaurs herbivores before the Fall of Man. Before the fall, everything was perfect but after the Fall, everything spiraled into what we know now: an imperfect world. That is when the dinosaurs supposedly became carnivores.
To find more out about this, check out Answers in Genesis!
Hope this helps, have a great day/night!